The invention relates to the design of a dipole antenna.
Dipole antennas have many commercial and military applications such as cellular telephones and other mobile communications and data links. An ideal dipole antenna has broad bandwidth, high efficiency, an unobstructed radiation path and minimal spurious radiation, and for mobile applications, can withstand mechanical vibrations.
Many types of dipole antennas, such as those illustrated in FIGS. 1-3, are known in the art. The antenna of FIG. 1 comprises quarter wavelength dipole radiators 10 and 12, balanced transmission-line conductors 14 and 16, a balun 18, a coaxial cable 20 and a drive signal source 22. The balanced conductors 14 and 16 are formed on opposite sides of a dielectric support sheet (not shown) to match the impedance of the coaxial cable and cancel unwanted radiations from the equal and opposite currents flowing through the balanced conductors. The balun 18 comprises a one quarter wavelength strip conductor which is electrically connected to the end of the inner conductor of the coaxial cable 20, extends parallel to the coaxial cable 20 and electrically connects to the shield of the coaxial cable 20 one quarter wavelength from the end of the coaxial cable 20. The quarter wavelength of the shield also serves as part of the balun. The principle of operation of a balun is well known in the art; a balun reduces standing wave ratio, assists in impedance matching and prevents high frequency currents from flowing onto the shield of the coaxial cable from the radiators via the balanced conductors. Without a balun, such currents would cause unwanted radiation from the shield. The coaxial cable 20 transmits the drive signal from the drive signal source 22 to the radiators 10 and 12 via the balanced conductors 14 and 16. While the overall design minimizes unwanted radiation, the lateral position of the balanced conductors 14 and 16, balun 18 and coaxial cable 20 relative to the radiators 10 and 12 interferes with one direction of lateral radiation of the radiators. Also, the cantilevered support of the radiators is undesirable from a mechanical standpoint.
The antenna of FIG. 2 is a basic coaxial dipole design (and was disclosed in U.S. Pat. No. 2,184,729). A quarter wavelength radiator 30 is connected to an inner conductor 32 of a coaxial cable 34. An upper end of outer conductor 36 of the coaxial cable is connected to a concentric metal sleeve 38 which serves as the other radiator. The metal sleeve radiator is also one quarter wavelength and therefore, acts as a choke and presents a moderately high impedance between its lower end and the outer conductor 36 of the coaxial cable. This minimizes unwanted RF current flow in the outer conductor 36 of the coaxial cable. However, the bandwidth is limited because of the impedance nature of the choke formed by outer conductor 36 and the sleeve 38. A variation of this design uses a one-half wavelength radiator corresponding to feature 30 of FIG. 2 (as disclosed in U.S. Pat. No. 4,352,109). This configuration requires an inconvenient impedance transformation to the asymmetrical dipole feed location.
The antenna of FIG. 3 is similar to that of FIG. 2 and comprises a radiator 40, a metal sleeve radiator 41 and a coaxial cable 42. However unlike the antenna of FIG. 2, the antenna of FIG. 3 also includes a discrete choke 44 between the metal sleeve radiator (which can alternately be the outer conductor of a triaxial cable) and a discrete capacitor 46 between the two outer conductors (which can be metal braids). While such a design increases the impedance at the center frequency, the bandwidth is limited because of the additional discrete reactive components. Also, the choke is undesirable because it requires extra fabrication steps and skillful tuning.
A general object of the present invention is to provide a dipole antenna for which the feed arrangement does not interfere with the radiation pattern.
Another general object of the present invention is to provide a dipole antenna of the foregoing type which has minimal unwanted radiation.
Another general object of the present invention is to provide a dipole antenna of the foregoing type which has sufficient bandwidth for many applications.
Another general object of the present invention is to provide a dipole antenna of the foregoing type with good mechanical support.
Another general object of the present invention is to provide a dipole antenna of the foregoing types which can be fabricated economically.